238 CHEMICAL INFLUENCES ON YEAST. 



fear that any disturbance of vinous fermentation will be caused by 

 the copper sprinkled on the grapes. According to TSCHIRCH (II.), 

 the results of E. Mach's researches confirmed by the investiga- 

 tions of M. HOFFMANN (V.) on Portugese wine only about one- 

 tenth of the copper on the grapes finds its way into the must, all 

 the rest remaining on the skins. Moreover, it seems from 

 Polacci's researches that only a small fraction of that tenth 

 actually becomes operative, so that but little damage can be 

 caused, and Chuard has shown that this soluble remainder is 

 mostly precipitated as malate and tartrate as the percentage of 

 alcohol increases during fermentation, some portion being also 

 converted into sulphide by the sulphuretted hydrogen (see vol. ii. 

 p. 200) produced by the yeast cells. Consequently the clarified 

 wine after separation from the yeast will only contain a few milli- 

 grammes of copper per litre, even when made from grapes that 

 have been extensively sprinkled with copper. Of course a corre- 

 spondingly larger amount will be present in the deposited yeast. 



From the researches of H. MANN (I.) and H. POTTEVIN (I.) it 

 may be concluded that the yeast converts a portion of the copper 

 sulphate in the nutrient solution into copper phosphates 

 (Cu 2 H 2 P 2 8 and Cu 3 P 2 O 8 ), whilst another portion is retained by the 

 cells. When a yeast thus enriched with copper is transferred to 

 a colourless nutrient solution, the observations of H. WILL (I.) 

 show that the metal (i.e., an unidentified compound of same) 

 passes into the liquid, which acquires a blue tinge. The combina- 

 tion is probably between the metal and substances of the nature 

 of those already referred to as yeast gum on p. 176, vol. ii. ; and 

 for this reason also, the figures already quoted above with regard 

 to the toxic action of copper sulphate cannot be regarded as un- 

 conditionally accurate. 



When pure yeast is grown in copper vessels for use in practical 

 fermentation, the inner side of the vessel in contact with the 

 fermenting liquid must always be well tinned. Otherwise the 

 copper will be corroded, and the yeast crop will contain appreciable 

 quantities of the metal, which is undesirable for several reasons. 

 Thus, H. SEYFFERT (I.) detected 0.27-0.64 per cent, of CuO in 

 the ash of pure yeast (see vol. ii. p. 196) grown in an apparatus that 

 was defective in this respect. This quantity probably consisted 

 in part of insoluble copper salts formed during fermentation and 

 deposited, and partly of cupriferous constituents of the yeast cells. 

 The tin used for plating the apparatus should be as low as possible 

 in lead, since, as observed by PRIOR (IV.), this latter metal injures 

 yeast considerably. 



268. Behaviour of Yeast Cells toward Alcohol. 



From the standpoint of the cecological theory of fermentation, 

 the alcohol produced by veast should be regarded as a weapon 



